The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the disclosure.
Traditionally, petroleum-based oils and greases have played a dominant role in applications requiring lubrication. However, with new technology needing lubrication in extreme conditions such as high temperature, high pressure, maintenance free systems, low-emission systems, and use in high vacuum [1], much emphasis has been placed in the development of effective solid lubricants which can be coated onto component's surfaces. Using recent advances in coating technology, it is now possible to apply thin layers of solid lubricants on surfaces, which changes both the chemical and physical properties of the surface. These films have the potential of being used for applications in microelectromechanical systems (MEMS), biomedical devices, and machine components in order to reduce energy losses due to friction, as well as reducing corrosion and surface fouling.
Polytetrafluoroethylene (PTFE), commonly known by its brand name Teflon, has particularly drawn much attention as a solid lubricant. It is attractive because of its self-lubricating properties, low coefficient of friction (COF), high temperature resistance, and chemical resistance [2,3]. However, PTFE is highly susceptible to wear, and as such, PTFE alone cannot be used in most applications. Because of this, many investigations have focused on creating PTFE composites that possess a greater wear resistance. PTFE composites with various micro and nanoparticle fillers such as glass fiber [4], alumina [5], and graphite [6], have been studied. These investigations have been carried out on bulk PTFE. Although there has been much progress made in the tribological study of bulk PTFE, there has been less focus placed on the tribological performance of PTFE thin films [7].
The wear behavior of PTFE films is not only characterized by local wear of the PTFE film itself, but also by the delamination of the film [8] resulting from weak adhesion of the film to the substrate. To increase the adhesion of film to substrate, investigators have used surface roughening techniques as well as primer coats to allow PTFE to physically lock and adhere to the surface. Primers such as polyamide acid [9] and fluorinated ethylene propylene/PTFE blends [10] have commonly been used for this purpose. To ensure durability, these films have typical thicknesses above 20 μm [9, 10]. The large thickness of these films, as well as the need for large peak-to-valley roughness on the substrate surface, limits their use in many applications where thin films are required.
In order to increase the wear resistance of PTFE thin films without the use of surface roughening and thick coatings, it is important to find a material with strong affinity to both PTFE and the substrate. Polydopamine (PDA) has been found to adhere well to many organic and inorganic materials, including PTFE [11]. PDA is synthesized through an oxidative reaction and is rich in 3,4-dihydroxy-L-phenylalanine (DOPA) and lysine peptides. Although the exact mechanism behind the adhesive property of PDA is not known, it is believed that the catechol functional groups in DOPA and amine in lysine play a significant role in the process [11]. These properties of PDA have only recently been discovered, and thus few studies have been completed on the tribological performance of PDA films.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies. This invention addresses the aforementioned deficiencies by utilizing a PDA adhesive layer to improve the tribological performance of a PTFE top coat. The results show a similar COF ranging between 0.04 and 0.06 for a PDA/PTFE film compared to a PTFE film alone. Due to the strong adhesion between PTFE and PDA, the PDA/PTFE film is able to withstand approximately 500 times more rubbing cycles than the PTFE film alone. A tenacious layer of PTFE remains strongly adhered to the PDA film, and contributes to the durability of the film [12].